It is said to help us prevent kidney damage, lose weight and increase concentration levels.

But experts now warn that drinking eight glasses of water a day is not good for you after all – and could be harmful.

They say that scientific claims behind long-standing government guidelines are worse than ‘nonsense’.

The NHS – along with leading doctors and nutritionists – advises the public to drink about 1.2 litres (or two-and-a-half pints) of water per day.

However, a report describes the danger of dehydration as a ‘myth’ and says there is no evidence behind claims that water prevents multiple health problems.

Glasgow-based GP Margaret McCartney says the NHS Choices website’s advice that people should drink six to eight glasses a day is ‘not only nonsense, but thoroughly debunked nonsense’. She adds that the benefits of the drink are often exaggerated by ‘organisations with vested interests’ such as bottled water brands.

Writing in the British Medical Journal, Dr McCartney also points out that research shows drinking when not thirsty can impair concentration, rather than boost it, and separate evidence suggests that chemicals used for disinfection found in bottled water could be bad for your health.

Drinking excessive amounts can also lead to loss of sleep as people have to get up in the night to go to the toilet, and other studies show it can even cause kidney damage, instead of preventing it.

Worryingly, Dr McCartney also warns that taking on too much water can lead to a rare but potentially fatal condition called hyponatraemia, which sees the body’s salt levels drop and can lead to swelling of the brain.

In 2003 actor Anthony Andrews, who starred in the ITV adaptation of Brideshead Revisited, was hit by the illness after drinking too much water during rehearsals for a West End role.

Another doctor quoted in the article adds there is no basis for claims that water helps people to lose weight by suppressing their appetite. Professor Stanley Goldfarb, a metabolism expert from the University of Pennsylvania in the U.S., says: ‘The current evidence is that there really is no evidence.

‘If children drank more water rather than getting extra calories from soda, that’s good . . . [but] there is no evidence that drinking water before meals reduces appetite during a meal.’

About 2.06billion litres of bottled water was drunk in Britain last year, compared with 1.42billion litres in 2000. Despite this increase we still drink three times as much tea, and five times as much beer.